The present invention relates to voltmeters generally and to voltmeters for use in electrical power transmission line servicing and maintenance in particular.
Electricity transmitted through power lines destined for commercial, industrial and residential use can involve hundreds of thousands of volts and high currents. Inevitably, there is an element of danger in measuring the voltage on a transmission line because of the need to make contact with the line. Indeed, even the proximity to a high voltage line may be sufficient to cause a spark to jump through the air to the nearest object. Nonetheless, in installing, servicing and repairing power lines, there are various occasions when contact is made, such as when the voltage carried by a line must be measured.
The circumstances and equipment used for measurements of the voltage of transmission lines varies considerably. For example, the absolute voltage carried by a line may be measured by a xe2x80x9chigh line resistive voltmeter.xe2x80x9d As another example, in servicing or repairing voltage regulators, an xe2x80x9coff neutral detectorxe2x80x9d is used to determine if the regulator is passing current or has been effectively isolated from the power source. In still another application, a xe2x80x9cphasing voltmeterxe2x80x9d is customarily used in connecting individual lines of the multi-phase transmission power lines. The phasing voltmeter helps to prevent two lines that are not in phase from being connected inadvertently.
Presently, high voltage phasing voltmeters use two test probes, which are each high voltage resistors housed in an insulated holder, in series with each other and a meter and a cable. The test probes are connected to the series cable and the series meter. The holders will have metal hooks or other fittings on their ends for good electrical contact with transmission lines. Often the meter is mounted to one of the two test probes and oriented so that the electric utility worker can read the voltage displayed on the meter. xe2x80x9cHot sticksxe2x80x9d may be used to hold and elevate the entire assembly. The meter may be designed to measure either voltage or current, but its display indicates voltage. However, the indicated voltage is not always the true voltage difference for the four types of measurements listed above.
High voltage measurements are plagued with inaccuracies stemming from stray capacitive charging currents. At high voltages, these stray currents emanate from the surface of every component of the measuring device including the cable. The capacitive current is related to the capacitive reactance, Xc, which can range from several thousand ohms on up, depending on the position of the meter and cable with respect to the ground. Under extreme conditions, such as when the series cable is lying directly on the ground between two pad-mounted transformers, the value of the capacitive reactance can be very low. The resulting capacitive current can then equal or exceed the measured current. Moreover, the voltage measured by the meter varies depending on the location of the meter and cable.
However, the inaccuracies in phasing voltmeters attributable to capacitive currents are eliminated by the design disclosed and described in a commonly owned U.S. patent application, Ser. No. 09/766,254, filed Jan. 18, 2001, which will be referred to herein as the xe2x80x9ccompanion specification.xe2x80x9d
Other problems with phasing voltmeters have not been solved. On occasion, the two power transmission lines are separated by a considerable distance. While the alternating current phasing voltmeter disclosed in the related application practically eliminates capacitive currents regardless of the length of the cable, it does not effectively address the problem of the logistics in dealing with a long cable or the problem of having a cable that is not long enough.
There are other problems with phasing voltmeters. When very high voltages are being measured, the inherent dangers of applying the insulated test probes with a connecting cable to the lines create a natural reluctance to proceed.
These devices are used in the out of doors, during all types of weather and at all times of the day; therefore, being able to extract the indicated voltage is not always easy to do or, in fact, is it done accurately.
Thus there remains a need for a phasing voltmeter that is accurate regardless of the capacitive current, easy to read, and can be easily used when the transmission lines are separated by more than a few feet.
According to its major aspects and briefly recited, the present invention is a wireless phasing voltmeter where the capacitive currents are combined with the primary voltage measurement of the electrical transmission lines in such a way that the capacitive current has no net affect on the voltage measured regardless of the magnitude of the capacitive current. The voltage signal is transmitted using radio frequencies from one test probe to the voltmeter and the output of the voltmeter may be displayed digitally. Thus, the meter can be applied to transmission lines a considerable distance apart without being limited by the length of a cable.
The present phasing voltmeter includes a pair of test probes in series with a high impedance alternating current (AC) voltmeter, a radio frequency transmitter, and a radio frequency receiver. In parallel with the voltmeter is a low impedance electrical circuit, tied electrically at a single point of contact to electrical shielding surrounding and electrically isolating the resistors, AC voltmeter, transmitter and receiver. The shielding picks up the capacitive currents in the vicinity of the phasing voltmeter.
The purpose of the electrical circuit is described in the companion specification. Furthermore, a number of different embodiments of this electrical circuit are described in the companion specification, but, for convenience, only a gain and a balance resistor will be illustrated and described for use with the present invention, however, it will be understood that any of those described in the companion specification may be used with the present invention,
In addition to the high impedance resistors, voltmeter, transmitter, receiver and electrical circuit, a phase shift network may optionally, but preferably, be added to account for the shift in the phase that occurs. Also, the present wireless AC phasing voltmeter may process and transmit the measured voltage digitally to provide an alternative way to transmit the signal from the second probe.
An important feature of the present invention is the use of radio frequency transmitter and receiver to transmit the signal from one of the high impedance resistors to the voltmeter. This feature has several advantages. First, it eliminates the cable, which, in addition to the cost and the requirement to manage it as part of the voltmeter, imposes a significant physical limitation on the distance between the two high impedance resistors. Second, it allows measurements of the voltage difference between two power transmission lines that may be very far apart. Third, it reduces the natural trepidation of workers who are responsible for making measurement on transmission lines carrying very large voltages. Although any electromagnetic waves (visible, infra-red, radio-frequencies, microwave, for example) can be used, radio-frequencies are preferred because they allow for other objects to be in the line of sight between transmitter and receiver without loss of signal. Preferably the signals are transmitted digitally and in such a way as to minimize the effect of electrical noise on the transmission, such as by frequency shift keying.
Another feature of the present invention is that current phasing voltmeters can be backfitted to use a transmitter and a receiver instead of a cable. Currently, the cable has jacks on each end that plug into the voltmeter and the other test probe. The present transmitter and receiver can be made with jacks that plug into the same plugs the cable would use.
Still another feature of the present invention is the inclusion of a phase shift network to eliminate a small error that is introduced by the modulation and demodulation of the electrical signal.
These and other features and their advantages will be apparent to those skilled in the art of transmission line voltage measurement from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings.